Nonmilitary aircraft utilize several radio links with the ground for business communications by crew or passengers. Pilots of business aircraft need to utilize these channels on a frequent basis for coordination of meetings, transportation, and other logistical functions because of their non-routine schedules. Even more often, the fast pace of their passengers, generally senior business and government executives, demands reliable and secure voice communication to maintain touch with their diverse organizations and activities.
Different radio types and frequencies are utilized for these functions since no single type provides communication in all geographical areas. Furthermore, strict governmental allocation determines the applications for which frequencies may be used. Thus, for example, although most civilian air traffic control is conducted over VHF (Very High Frequency) radios, additional frequencies in these same bands are used as "company" channels for exchange of operational messages such as those regarding schedules or ground transportation requirements. For this type of radio using one frequency for both parties, each must push a button on their microphone when speaking to turn on the transmitter (Push-to-talk or PTT). Following each transmission, they must then release the PTT to relinquish the frequency for the other party to respond. This process is known as simplex operation.
For longer range operation necessitated by remote area or over-water flights, a second set of radios using the HF (High Frequency) band must be switched into the crew's audio systems of microphones and headphones. Here again, the operation is simplex and separate frequencies are assigned for differing requirements. Frequently, marititime channels are used to call commercial ground stations which then tie the aircraft transmissions into international public switched telephone networks. Thus long range links may be established between the remote business traveller and almost any telephone in the world.
Next, a unique air-to-ground radiotelephone network is available within the Continental United States, Southern Canada, and Northern Mexico. This includes almost one hundred ground stations using UHF (Ultra High Frequency). Unlike the more common simplex VHF and HF radios, this system allows both parties to speak simultaneously--full duplex operation. The party on the ground transmits continuously on one frequency while the party in the aircraft transmits continuously on a second frequency. UHF is the communication link most used by the passenger today. In the near future, however, new links including satellite relay will be established for telephonic communication to the aircraft. It is desirable that any system addressing the multiple communication links existing today be readily adaptable to such new, full duplex links as they become available.
Thus, wide ranging business aircraft require a diverse suite of communication radios with differing technical characteristics and interface requirements. Although these several different communication links must be frequently utilized by most business jet aircraft, none permit private conversations. That is, all conversations, no matter how sensitive their nature, may be monitored by any party purchasing commonly available commercial receivers, a reality that exposes the users to potential hazard. For example, schedule coordination for significant public figure passengers often require broadcasts of movements which may be easily intercepted by terrorist organizations or others with even a minimum of technical sophistication. Moreover, the press of decisions frequently requires radiotelephone discussions by passengers of sensitive information which can be extremely detrimental to the speakers' organizations if received by interested outside parties.
Numerous technologies and devices exist which permit disguising or encrypting voice and data communications over any one of these channels. Typically, a device to scramble, distort, or in some other fashion rearrange audio frequency energy into an unrecognizable presentation, is installed between the microphone and transmitter input of each channel. Similarly, audio coming from the receiver paired with that transmitter is routed through a decryption unit before being carried to the airborne listener.
Since multiple channels are utilized in these aircraft operations, one solution to providing the necessary protection would be to install multiple and different encryption systems on board the aircraft which are appropriate to the individual link characteristics, voltage levels, and impedances. However, as aircraft are of necessity extremely sensitive to additional weight or power consumption, this is not a solution for any but the largest commercial aircraft. The cost of such duplicated equipment and its installation is significant, particularly since redundant radios might be required to provide a separate channel for passengers in order to avoid sharing all discussions with the flight crew. In business aircraft, the executive passengers are frequently the primary users for private radio-telephone channels, yet the crew is responsible for all radio transmissions and should maintain ultimate control over such security functions. In a typical business aircraft, the audio input and output of the radio are typically routed in common to both the cockpit and cabin telephone handsets. Thus, although the passenger's communications may be protected from interception by persons outside the aircraft, the aircraft crew will be able to eavesdrop. Therefore, means are needed to prevent the crew from eavesdropping on the communications.
In summary, there is a broad and present need for equipment (1) to apply high security encryption processes to all the diverse communications channels of business aircraft in the smallest possible size and weight configuration; (2) to provide passenger control over radiotelephone encryption when appropriate while maintaining the flight crew's ultimate control over such usage; and (3) to provide separate and private audio channels for crew and passengers as necessary while permitting shared communication channels when desired.